1. Field of the Invention
The present invention relates to a process for preparing compositions of the formula REBa wherein REBa.sub.2 Cu.sub.4 O.sub.y is a rare-earth element, including yttrium and dysprosium and subscript "y" is about 8.0. These compositions have particular utility in superconductors. The process of the instant invention utilizes nitric acid in the preparation of the compositions. The present invention further relates to a process of preparing a superconductor from these compositions.
2. Background of the Invention
Recently, REBa.sub.2 Cu.sub.4 O.sub.y (hereinafter referred to as RE-124, where RE is yttrium and/or dysprosium (Dy) and subscript "y" is about 8.0, has competed with REBa.sub.2 Cu.sub.3 O.sub.y 0 (hereinafter referred to as RE-123, where RE is yttrium and/or dypsrosium) as one of the most important superconductors being studied. Particular interest has focused on the advantages RE-124 has over RE-123. For example, although RE-123 generally manifests a higher superconducting temperature (Tc, generally expressed in units of Kelvin, K) than RE-124 (about 90K for RE-123, and about 80K for RE-124), it suffers from having high chemical reactivity as well as poor thermal stability. Thus when heated or stored for long periods of time, RE-123 decomposes, which decomposition results in a reduction in oxygen content and, consequently, a lowering of the superconducting temperature.
In contrast, RE-124 exhibits no such chemical reactivity and further has superior thermal stability due to its particular crystalline structure. Indeed, RE-124 will not decompose even at elevated temperatures, such as when heated up to 850.degree. C. As a result RE-124 does not suffer a loss of oxygen with the attendant depression of the superconducting temperature, which behavior is important for practical applications. Moreover, the superconducting temperature of RE-124 can be raised to about 90K --the approximate superconducting temperature of RE-123--by doping with a suitable amount of calcium. So marked are the advantages inherent in RE-124 that even its use as a precursor, i.e., a decomposition precursor, in the preparation of RE-123 has benefits: products thus made will exhibit higher hysteresis, as well as higher critical current density. Furthermore, RE-123 prepared using RE-124 as a decomposition precursor exhibits shielding effects which are superior to those exhibited by RE-123 prepared by any other process.
The known processes for manufacturing RE-124 high Tc superconductors fall into two general categories, namely: a high oxygen pressure process and an ambient oxygen pressure process. Literature related to the high oxygen pressure process include:
a) J. Karpinski, et al., Nature, 336, Dec. 15, 1988, pp. 660-662 report the synthesis of bulk RE-124 phase in oxygen at a pressure of 400 bar and a temperature of 1,040.degree. C. PA1 b) D. E. Morris, et al., Phys. Rev. B., 39, No. 10, pp. 7347-7350 (1989) report that an RE-124 (more particularly, Y-124) was sintered in high pressure oxygen (the pressure of O.sub.2 being approximately 120 atmospheres) at 930.degree. C. for 8 hours using a commercial high-pressure oxygen furnace. PA1 c) T. Miyatake, et al., Physica C., 160, pp. 541-544 (1989) describe a sample preparation of RE-124 by a solid state reaction method using the oxygen-HIP treatment. Starting materials were calcined at 900.degree. C. in flowing oxygen for 12 hours. The powder compact was sintered at 800.degree. C. in flowing oxygen. The oxygen-HIP treatment was repeated twice in an argon-oxygen gas environment (Argon +20% oxygen) at a pressure of 100MPa. The first treatment was at 950.degree. C. for 6 hours; the second at 1050.degree. C. for 3 hours PA1 a) R. J. Cava, et al., Nature, 338, pp. 328-330 (1989) report a method of using Y(NO.sub.3).sub.3 xH.sub.2 O, Ba(NO.sub.3).sub.2 and Cu(NO.sub.3).sub.2 xH.sub.2 O as starting materials which, after slow preheating and pulverizing, are mixed with an approximately equal volume of Na.sub.2 CO.sub.3 or K.sub.2 CO.sub.3 to catalytically enhance the reaction rate. The carbonate mixture is ground and heated at 800.degree. C. for 3 days. However, the hydrated nitrates are hygroscopic, making the control of the stoichiometry difficult. In addition, impurities can be introduced by adding Na.sub.2 CO.sub.3 or K.sub.2 CO.sub.3, thus complicating the whole process. PA1 b) S. Jin, et al., Physica C., 165, pp. 415-418 (1989) describe a synthesis route using YBa.sub.2 Cu.sub.3 O.sub.7 (the formula corresponds to RE-123 where RE is yttrium: Y-123) as a precursor. The Y-123 was mixed with CuO. Sintering was at 810.degree.-830.degree. C. for 3 days and was repeated 3-4 times. The disadvantages of this process include the need for a Y-123 precursor and the lengthy sintering time. PA1 c) D. M. Pooke, et al., Phys. Rev. B., 41, No. 10 pp. 6616-6620 (1990) report a process which comprises mixing stoichiometric proportions of Y.sub.2 O.sub.3, Ba(NO.sub.3).sub.2, and CuO with up to a 0.2 mol fraction of NaNO.sub.3 or KNO.sub.3 ; prereacting the mixture as a loose powder for 30 min; then grinding, die pelleting and reacting at 800.degree. C. for at least 12 hours in flowing oxygen. Phase purity is improved with repeated grinding and sintering. The disadvantages of this process include the use of alkali nitrates which are known to introduce impurities and the lengthy sintering time.
The disadvantages common to these methods include the need for a high pressure oxygen furnace and the need for reaction temperatures over 900.degree. C. These requirements increase production costs and are unfavorable for mass production, thus making the high oxygen pressure process unattractive for practical applications.
Literature related to the ambient oxygen pressure process include:
Thus despite intensive research efforts, the problems attendent current methods of preparing RE-124 demonstrate that a need for a more efficient process exists.